AssertionError on domains/c.py

[inconstxpr]

Describe the bug
I have set up a CMake project that generates documentation for C using doxygen, breathe and sphinx, following this guide. The sphinx target depends on sphinx/index.html, which can be generated by a custom command that in turn, depends on doxygen/xml/index.xml.

When using Visual Studio, Build > Clean All deletes all files that are generated by the build, in this case doxygen/xml/index.xml and sphinx/index.html. After that, the next build will fail with:

Exception occurred:
    File "c:\python39\lib\site-packages\sphinx\domains\c.py", line 1532, in _fill_empty
      assert line is not None
  AssertionError

After getting the error, running Build > Clean All and then another build works without problems. It alternates between the two, so the next cycle will give the same error. This is not exclusive to Visual Studio.

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/inconstxpr/icanc.git
$ cd icanc
$ mkdir build
$ cd build
$ cmake ..
$ cmake --build . --target sphinx # works
$ rm docs/doxygen/xml/index.xml
$ rm docs/sphinx/index.html
$ cmake --build . --target sphinx # error
$ rm docs/doxygen/xml/index.xml
$ # sphinx/index.html was not generated
$ cmake --build . --target sphinx # works

Expected behavior
The build should work every time.

Your project
https://github.com/inconstxpr/icanc.git

Environment info

• OS: Windows 10
• Python version: 3.9.0
• Sphinx version: 3.3.0
• Sphinx extensions: breathe
• Extra tools: CMake 3.19.0

[jakobandersen]

It looks like this could be the same problem reported and fixed in #8364. Try upgrading to a newer version of Sphinx. If it doesn't fix it, please reopen the issue.